Professional and skilled golfers favor golf balls using balata rubber as the cover stock because the balata cover offers a soft feel, a desired spin rate and control upon iron shots.
The golf balls using balata rubber, however, are rather difficult to mold and require an increased material cost and manufacturing cost. Because of low resilience, balata rubber is believed unsuitable for combination with solid cores. Another problem of the balata cover balls is that upon iron shots, the cover surface is scraped off by grooves across the iron clubface owing to the frictional force between the clubface and the cover, so that the ball surface is marred or fluffed.
Ionomer resin covers have been proposed. Sullivan, U.S. Pat. No. 4,884,814 or JP-A 308577/1989 discloses to blend an ionomer resin in the form of an ethylene/(meth)acrylic acid copolymer having a certain spectrum of physical properties with a relatively soft ionomer resin in the form of an ethylene/(meth)acrylic acid/(meth)acrylate terpolymer. The soft/hard ionomer blend is used as a golf ball cover. JP-A 277208/1993 discloses a golf ball using a mixture of two or more metal salts of ethylene-unsaturated carboxylic acid-unsaturated carboxylate terpolymers having a low flexural modulus as the cover stock. These are very effective techniques for achieving significant improvements in productivity and cost while maintaining hitting feel and controllability comparable to the balata rubber.
The ionomer resin covers of these patents, however, still suffer from the problem that the ball surface is marred or fluffed by iron shots because the cover surface is scraped off by grooves across the iron clubface. Additionally, on account of low resilience, the ionomer resin covers invite a substantial drop of resilience when combined with solid cores.
Many attempts were made to use thermosetting polyurethane elastomers as a substitute for the balata rubber and ionomer resins because the polyurethane elastomers are relatively inexpensive and offer good feeling and good scuff resistance. See U.S. Pat. Nos. 3,989,568, 4,123,061, and 5,334,673. Despite improvements in the scuff resistance which is the drawback of softened ionomer resin blends, the thermosetting polyurethane elastomers require complex steps of introducing the raw material and then effecting curing reaction, indicating that further efforts must be made for mass-scale production.
Also, U.S. Pat. Nos. 3,395,109, 4,248,432 and 4,442,282 disclose thermoplastic polyurethane elastomers as the cover stock. They do not satisfy all the requirements of moldability, hitting feel, control, resilience, and scuff resistance upon iron shots.
In this regard, we proposed in JP-A 271538/1997 the use of high resilience thermoplastic polyurethane elastomers. A further improvement in scuff resistance upon iron shots is desired.